What’s happening?

I have this Rex Begonia Paso Doble. Over the past couple months, the green part of the leaf is not greening. I can’t tell if she’s trying to yellow or brown, but she’s not the vibrant green she was when I got her. The bottom leaf is older and on the way out, but upper leaves just look… bland.

I have it under T5 Barrina lights in a IKEA Rudsta. I’ve started feeding with each watering – I use We the Wild Grow Concentrate. I usually water weekly, or when the top part of the soil is a little dry. The ambient humidity ranges between 50-60%

Any ideas?